Phosphorus is a major mineral nutrient essential to the growth and reproduction of photosynthetic organisms. Plants grown under natural soil conditions often experience sub-optimal growth because of inorganic phosphate (Pi) deprivation. Because crop productivity is strongly influenced by the availability of phosphorus, phosphate fertilizers are used to supplement the natural soil levels. However, excess phosphorus upsets the natural balance of aquatic and terrestrial ecosystems which border agricultural lands. Further, it is estimated that the mineral sources of phosphorus for the production of fertilizers will become severely depleted within fifteen years. Even a modest increase in the efficiency of plants to utilize phosphorus would make a significant contribution to the reduction of agricultural expenditures by producers and lower the impact on untilled ecosystems.
Phosphatase activity (orthophosphate-monoester phosphohydrolase) has been observed in all plants and in all tissues. Phosphatases are involved in the routine turnover of Pi from the many intracellular sources of orthophosphate esters (Hollander, 1971). Certain phosphatases exhibit phosphate-stress induction (psi) and are implicated in Pi acquisition through the release of Pi from the environment or from intracellular sources. Typically, these psi phosphatases are abundant and active toward a broad range of substrates. In contrast, the expression of phosphatases involved in cellular and metabolic regulation is generally discrete and tightly regulated.
Defined according to their pH optima, there are two types of phosphatases. The acid phosphatase (APase) and alkaline phosphatase subclasses are structurally distinct and undergo different reaction mechanisms (Kim and Wyckoff, 1989; Neuman, 1968). Alkaline phosphatases tend to demonstrate specificity toward a particular substrate whereas APases usually have no absolute substrate preference.
A major response to phosphate-deficiency involves the induction of phosphatase activity. Because the phosphoesters or polyphosphates found in soil cannot directly be assimilated by plant roots, Pi must be released from these compounds by phosphatase-mediated hydrolysis. Under phosphate-deficient conditions, increased acid phosphatase activity has been demonstrated in a number of plant species (Lefebvre et al, 1990; Dracup et al, 1984; Lefebvre and Glass, 1982). APases that respond in this manner are produced de novo and are secreted to the external cell wall environment in roots (Szabo-Nagy et al, 1987) and cell cultures (Duff et al, 1991a; Lefebvre et al, 1990). Controlling the expression of plant APases could result in crops with an enhanced capacity for phosphate uptake and lower fertilization requirements.
APases have been identified in every plant species tested and a number have been characterized (Duff et al, 1994). A species survey using antibodies raised against a Brassica nigra APase suggests a major family of closely related APases exists among plants (Duff et al, 1991b). A plant APase gene has been isolated that codes for a storage protein-related phosphatase (Erion et al, 1992); however, this enzyme is distinct from the major family of APases in both structure and function. No genes encoding members of the major family of plant phosphatases have been reported.
Detailed knowledge of the plant phosphatases which affect the uptake and metabolism of phosphorus is essential to understand phosphate metabolism and to manipulate the growth and reproduction of photosynthetic organisms for commercial or industrial purposes. Further, the identification and synthesis of the genes which encode plant phosphatases would allow the development of transgenic photosynthetic organisms for many purposes.